For the representation of a transmission ratio stage of an automatic transmission, several elements of a planetary transmission are connected to the housing or to each other in a torque-proof manner, which takes place by means of shifting elements formed as brakes or couplings. Thus, a different combination of elements is closed with different transmission ratio stages. A gearshift, which is also known as a change in the transmission ratio stage or below only as a shift, takes place in a specified shifting time through a switching off or opening, or switching on or closing, of specified shifting elements. The closing of the shifting elements takes place by hydraulic pressure or a different force effect. In the case of a frictional-locking hydraulic shifting element to be switched on, for reasons of shifting comfort, the transfer capacity of the shifting element correspondingly increases through a pressure increasing with a specified function over time, or decreases in the operation of the slip of the frictional-locking shifting element. With a frictional-locking shifting element, one speaks of a closed state if the shifting element halves are connected to each other in a torque-proof manner. With a frictional-locking shifting element to be switched off, the pressure and thus the transfer capacity decrease in a corresponding chronological progression. In the open state of a frictional-locking shifting element, the shifting element halves are separated from each other, such that, except for a drag torque, a torque sufficient for the propulsion of the vehicle can be transferred.
For an automatic transmission with hydraulic frictional-locking elements, the shifting element to be switched on (and thus to be closed) is initially prepared (i.e., filled) for the assumption of torque. Likewise, preparatory measures for the switching off shifting element, such as a reduction of the pressure to a level at which there is a defined slip-afflicted transfer capacity, may also be taken. Typically, the preparatory filling process of a shifting element is split into a rapid filling phase and a filling adjustment phase. During the rapid filling phase, the shifting element is filled with oil, whereas, the piston is applied with low pressure in the filling adjustment phase, such that the clearance in the shifting element (for example, formed as a multi-disk coupling) is raised, but a sufficient torque for the drive or the bracing of the automatic transmission is still not transferable. Through the preparation of the switching on of the shifting elements and the preparation of the switching off of the shifting elements, delay periods arise, in which the change to the rotational speed does not continuously run to the next transmission ratio stage, but remains at the synchronous rotational speed of the previous shift. This results in a perceptible stage in the change to the rotational speed, which also has negative effects on shifting comfort.
The constantly increasing requirements on the functionality of automatic transmissions through the demand for more spontaneity, the ever-growing number of transmission ratio stages or gears to be switched on, the consumption-optimized design of automatic transmissions with the larger driving-mode shares in the high gears and the number of downshifts to be carried out, which has grown with the number of gears, and for the braking of the vehicle up to a stop require that the gears of an automatic transmission should be shifted one after the other with greater and greater speed and frequency.
For example, upon braking processes, depending on the difference in speed to be cut down, it may be necessary to undertake a change of transmission ratio stages over several transmission ratio stages, whereas, at the beginning of the braking process, the known target transmission ratio stage is not yet known based on the output transmission ratio stage, such that this cannot be directly engaged. In addition, if even for the target transmission ratio stage, but not for all transmission ratio stages, it is possible to shift directly in it, it is necessary to open only one shifting element in a shifting process and to close another shifting element, in order to avoid interruptions in the pulling force.
DE 100 35 479 A1 shows a method for operating an automatic transmission with frictional-locking shifting elements, with which shifts that are consecutive in the same shifting direction, i.e., either multiple upshifts or multiple downshifts one after the other, do not take place as a multiple shift, but are interlaced within each other. In this context, a multiple shift is understood to be a multitude of successive shifts, whereas the next shifting process starts only after the preceding shift is completed and the corresponding transmission ratio stage is engaged. An interlacing of shifts is understood such that the subsequent shifting process is already prepared during the first shifting process. The preparation of a shift is understood such that the shifting elements to the switched on upon the next shifting process are pre-filled, and are thus closed immediately after reaching a synchronous rotational speed of the first shift. In particular, the interlaced shift is the case of a change of transmission ratio stages, with which more than one shifting element is open, and thus more than one shifting element is closed.
Through the interlaced shift, it is possible to undertake one shift over multiple transmission ratio stages without an interim stage being engaged, as with the multiple shift. With the interlaced shift, the shifting time from the disengaging of the output transmission ratio stage up to the engaged condition of the last transmission ratio stage is significantly shortened compared to the multiple shift. It is also possible to start a shift from an output transmission ratio stage into a first target transmission ratio stage and, in the case of the requirement of a second target transmission ratio stage in the same direction through the prepared shifting elements, to have the option of spontaneously engaging the second target transmission ratio stage with a short shifting time. In addition, the interlaced shift can also be cut off, and the first target transmission ratio stage can be engaged, if this is necessary.
It is known that frictional-locking shifting elements selected with the goal of improving the efficiency of automatic transmissions are replaced with positive-locking shifting elements. Advantageously, the drag torque of the positive-locking shifting elements in their open state is significantly less than the drag torque of an open frictional-locking shifting element. For this reason, with a known transmission, certain frictional-locking shifting elements are replaced with positive-locking shifting elements. DE 10 2008 000 429 A1 shows such an automatic transmission, which includes four frictional-locking shifting elements and two positive-locking shifting elements. Thereby, only such shifting elements that are disengaged upon upshifts may be formed to be positive-locking; i.e., the engagement of the positive-locking shifting element takes place only for downshifts. Thereby, a disengagement of the positive-locking shifting element is understood to be a process for which the shifting element halves are separated. At the beginning of the process, the shifting element is still closed and, at its end, the shifting element is open, by which a transfer of torque can no longer take place. As an analogy to this, the insertion or engagement of a positive-locking shifting element is understood as a process with which a positive-locking shifting element is, starting from an open state, closed.
If, for an interlaced shift, a shifting element to be switched on is formed to be positive-locking, for example as a claw coupling, which is to be switched on as early as the first change to the transmission ratio, according to the known method, this is to be closed at the point in time at which the original frictional-locking shifting element takes over the transfer of torque. Thereafter, an additional frictional-locking shifting element to be switched on, which was prepared for the first shift, is closed.
In order to be able to engage a positive-locking shifting element, the differential rotational speed of the shifting element halves must not exceed a specified maximum value, since damages to the form elements, such as claws or gear teeth, will otherwise arise. It is also possible that the claws of the two halves of the coupling do not fully engage in each other. If the differential rotational speed is too small, or the rotational speed of the two halves of the coupling are the same, the claw gear teeth during the engagement process can occupy a tooth-to-tooth position, which makes an engagement of the gear teeth in each other impossible. For this reason, it is necessary to, using the knowledge of the input rotational speed and output rotational speed of the automatic transmission, determine the differential rotational speed of the shifting element halves and, through measures such as the short-term influencing of the engine rotational speed or the braking of one half of the shifting element, reduce it through the friction torque of a frictional-locking shifting element into the desired range. The rotational speed of the turbine wheel of the hydrodynamic converter, and not the engine rotational speed, is thereby used as the input rotational speed, since differences between the engine rotational speed and the transmission input shaft may occur due to a possible slip of the hydrodynamic torque converter.
However, with a known interlacing of the shifts, it is disadvantageous that this determination of the differential rotational speed is not clearly possible, since, simply in view of the subsequent shift, a frictional-locking shifting element to be switched off was, in its transfer capacity, reduced at least to one slip-afflicted area, and a frictional-locking shifting element to be switched on is still not fully closed. Thereby, upon an attempt to engage the positive-locking shifting element, in addition to the damage already mentioned, torque shocks may arise if the rotational speed difference is too large. This results in torque shocks and thus poor shifting comfort.